1. Field of the Invention
The present invention relates to an image-forming method and an image-forming apparatus to be used for an electrophotographic system.
2. Description of the Related Art
In the image-forming method in an electrophotographic system, an electrostatic latent image is formed on a photosensitive member by an exposure device, the electrostatic latent image is developed by using toner, and the toner image is transferred on to a recording member such as paper and an OHP sheet, and the transferred image is fixed by a heating means or the like to obtain an outputted object.
In recent years, there have been strong demands for high resolution and high gradation in copying machines and laser printers. In order to satisfy these demands, the optical system, transfer speed and the like have been improved. However, in the conventional developing system using toner, characters and images on the resulting outputted object tend to lack sharpness and a sufficient gradation property, resulting in problems of disconnected highlighted portions in a photographic image and damaged shadow portions. For this reason, there has been the necessity of sharpening the particle-size distribution by making the toner particle size smaller.
Conventionally, the toner has been manufactured through a so-called pulverizing method in which, after a pigment such as carbon black has been mixed, fused and kneaded in a thermoplastic resin to be formed into an uniformly dispersed matter, this is pulverized by an appropriate fine pulverizing device into particles having an appropriate particle size required as a toner. In the pulverizing method, the shape of toner becomes indefinite, which is not necessarily appropriate for high-resolution and high-gradation. Since a classifying process is required so as to control the particle-size distribution, the costs become higher, and there is a limitation in efficiency in providing a smaller particle size.
Therefore, in recent years, from the viewpoint of reduction in the manufacturing costs and high image quality, granulation methods in a wet system, typically represented by a suspension polymerizing method and an emulsion dispersing method, which can provide resin fine particles having a small, comparatively uniform particle size, have received much attention in place of the pulverizing method.
In the suspension polymerizing method, a polymer composition having components, such as a polymerizable monomer, a polymerization initiator and a coloring agent, is suspended in a dispersion medium, and polymerized so as to carry out a granulation process. The toner obtained through the suspension polymerizing method provides resin particles after the polymerization that directly have a particle size suitable for toner particles, and the shape thereof has a virtually true spherical shape. The toner manufactured through the suspension polymerizing method is poor in its cleaning property on the photosensitive member, and has difficulty to be sharply controlled regarding the particle size distribution, although it is suitable for preparing high-quality images. In the case when the cleaning property is poor on the photosensitive member, when residual toner on the surface of the photosensitive member is cleaned by using a cleaning blade, toner escape from the blade tends to occur, resulting in filming on the surface of the photosensitive member and the subsequent deteriration in the image quality of the resulting image.
In the emulsion dispersing method, a binder resin and a coloring agent are dissolved or dispersed in an appropriate organic solvent to prepare a colored resin solution. After adding the solution to an aqueous dispersion solution, the resulting solution is stirred hard so as to form droplets in the resin solution, and heated to remove the organic solvent from the droplets so that a granulating process is carried out. With respect to the toner obtained from the emulsion dispersing method, it is possible to obtain toner having a small particle size by properly selecting processing conditions, and also to obtain toner having an indefinite shape; however, it is difficult to sharply control the particle size distribution.
Conventional problems with photosensitive members include a problem with abrasion resistance in which an abrasion occurs in the photosensitive layer due to long-term use, a problem with transferring property in which one portion of a toner image formed on the photosensitive member is not copied onto a copying material to cause an image loss, and the above-mentioned problem with a cleaning property. In particular, upon application of toner having a small particle size that is effectively used for obtaining a high-precision image (images with high resolution and high gradation), the deterioration in the cleaning property becomes serious. Although high resolution and high gradation in the initial image can be achieved, a new problem is raised in which upon continuous printing processes, there is deterioration in the resolution and gradation. In an attempt to prevent such deterioration in the resolution and gradation occurring upon continuous printing processes and to obtain stable and desirable images, it is necessary to improve not only the photosensitive member, but also both of the member and the developer.